Gamma-Fe.sub.2 O.sub.3 magnetic pigments have coercivity of up to about 31 kA/m, ferrite magnetic pigments up to about 35 kA/m. Magnetic pigments with a much higher coercivity are increasingly used for the magnetic recording of signals with a greater storage density in the audio-, video- and data-storage fields. Various processes are known for increasing the coercivity of magnetic iron oxides.
According to DE-A 29 03 593, pigments with higher coercivity are obtained from alpha-FeOOH precursor compounds doped with cobalt. A coercive field strength of about 50 kA/m is obtained by using 1% cobalt and an FeO content of 20%. Disadvantages found in these pigments are the low print through value, insufficient magnetic stability and high dependence of the coercivity on the temperature.
Another method of obtaining high coercivity is described in DE-A 22 35 383, according to which a layer of cobalt ferrite is crystallised epitactically on a core of magnetic iron oxide in a strongly alkaline medium. These pigments have better print through and better magnetic stability than cobalt doped pigments but are not entirely satisfactory in their switching field distribution and in the aging erasability obtainable in tapes.
Processes for the preparation of magnetic iron oxide pigments with improved magnetostatic properties are described in DE-A 20 36 612, DE-A 22 43 231, DE-A 24 10 517 and DE-A 22 89 344. In these processes, a cobalt compound, generally cobalt hydroxide, is precipitated on the surface of the iron oxide core material. Suitable measures are then carried out to cause the cobalt to diffuse into the near-surface zones of the core material.
Processes in which compounds of cobalt and iron are precipitated separately in several layers on the iron oxide core material to improve the magnetostatic properties are described in DE-A 35 20 210, DE-A 36 31 193 and DE-A 33 44 299.
Although the magnetostatics of the pigments may be improved by using some of the processes mentioned above, all these processes have the disadvantage that a combination of high coercivity with good switching field distribution can only be achieved by carrying out a heat treatment above 100.degree. C., which has an adverse effect on the magnetic stability of the oxides.
The problem therefore arose of finding an improved process for the preparation of magnetic iron oxide pigments which would be free from the disadvantages described above.